The invention relates to an electronic circuit intended to feed a coil sending out a magnetic field, having a first input terminal for receiving a power-supply voltage, a second input terminal for receiving a periodic control signal in a predefined frequency range, an output terminal for generating an output current in the said sending coil connected between the said output terminal and earth, in such a way as to convert the said periodic control signal into a periodic magnetic field sent out by the coil.
Such a circuit is intended particularly to feed a magnetic-field sending coil forming an antenna in a system, called xe2x80x9chands-freexe2x80x9d system, for access to an enclosed space, this enclosure possibly being a motor vehicle, for example. Such a system may also serve to allow or prevent a vehicle being started. Such a system generally includes a recognition device having an antenna in the form of a coil, which sends out a periodic magnetic field in order to carry on an exchange of data with an identification unit to be authenticated. For this application, the two characteristics of this magnetic field which are of use are its frequency and its radiation pattern. Conventionally, the circuit for feeding the sending coil receives a control signal at a given frequency and applies to the sending coil a voltage age which has the frequency of the control signal and which has as its amplitude the amplitude of the voltage of the battery of the vehicle.
In a vehicle equipped with a 12-volt-type battery, for example, the power-supply voltage of the battery may fluctuate between 10 and 16 volts; thus, when these fluctuations have direct repercussions on the amplitude of the output voltage which is applied to the sending coil, corresponding fluctuations ensue in the range of the magnetic field. These fluctuations are prejudicial, since it is desired that the authentication of the identification unit can always be carried out under standard conditions, such as a minimum standard distance between the user and the vehicle, for example.
This problem can be remedied by integrating a voltage regulator into the feed circuit of the sending coil, in order to have a radiation pattern of the magnetic field the shape of which does not vary as a function of the fluctuations in the power-supply voltage. The defect in this solution is the increase in the cost of manufacture of the feed circuit.
The object of the invention is to remedy these drawbacks.
To that end, the subject of the invention is an electronic circuit intended to feed a coil sending out a magnetic field, having a first input terminal for receiving a power-supply voltage, a second input terminal for receiving a periodic control signal, an output terminal for applying an output voltage to the terminals of the said sending coil, so as to convert the said periodic control signal into a periodic magnetic field sent out by the coil, characterized in that the said output voltage is a periodic signal having a period identical to the period of the control signal and a duty cycle which depends on the power-supply voltage, so that a current flowing in the coil has a peak intensity corresponding to a reference peak intensity.
With such a configuration, the feed circuit manages the peak intensity of the current which passes through the sending coil so that it is always equal to a reference peak intensity; thus, the range of the magnetic field sent out by the coil does not fluctuate as a function of the variations in the power-supply voltage supplied by the battery, and this without having to integrate a voltage regulator into the feed circuit
In one preferred embodiment of the circuit according to the invention, the reference peak intensity is adjustable, which makes it possible, for example, to alter the range of the magnetic field sent out by the coil in order more finely to evaluate the physical location of an identification unit to be authenticated in the course of a data exchange.